The present invention relates generally to optical gratings, and more particularly, to systems methods and apparatus for tuning and controlling bandwidth by bending an optical grating to select a bandwidth of wavelengths of light centered on a selected center wavelength.
Gratings are commonly used to select a narrowed light beam. However, the bandwidth of wavelengths in the narrowed light beam is not as easily selectable with a typical grating. FIG. 1 is a simplified schematic of a typical light beam narrowing system 100. The typical light beam narrowing system 100 includes a source light beam 102, directed through a beam expander 104 (typically including one or more prisms), and a grating 106. The grating 106 has a reflecting surface 108 with many grating lines 110. The reflecting surface 108 has a curve 112 substantially equaling a wavefront curve 114 of the expanded source light beam 102A. It should be noted that the components 102-114 of the light beam narrowing system 100 are not drawn to scale and specifically the curve 112, wavefront 114 and the pitch of the grating are exaggerated for exemplary purposes.
The expanded source light beam 102A includes multiple wavelengths 116A-n of light. The multiple wavelengths 116A-n of light are diverging at different angles relative to the beam expander 104 and impinge on the reflecting surface 108 in corresponding different locations.
Ideally, a selected grating line 110A reflects a narrowed light beam 124 including only the corresponding reflected wavelength 116E′ toward the beam expander 104 at the appropriate angle 118 such that the narrowed light beam 124 passes back though the beam expander 104 to the optical system 120 beyond the beam expander 104. Unfortunately, the selected grating line 110A also reflects a bandwidth of wavelengths including slightly shorter wavelengths 1502A than the reflected center wavelength 116E′ and slightly longer wavelengths 1502B than the reflected center wavelength 116E′. Thus the narrowed light beam 124 includes the reflected center wavelength 116E′ and the bandwidth of wavelengths including slightly shorter wavelengths 1502A than the reflected center wavelength 116E′ and slightly longer wavelengths 1502B than the reflected center wavelength 116E′.
Tuning the beam expander 104 and the amount of curvature in the curve 112 allows for a very precise center wavelength selection and a very narrow maximum bandwidth, e.g., less than 1.0 pm (1.0×10−12 meter)+/−either side of the reflected center wavelength 116E′, for the narrowed light beam 124. However, tuning the beam expander 104 does not allow for accurate control or selection of both a maximum bandwidth and a minimum bandwidth for the narrowed light beam 124, e.g. a bandwidth between 0.5 to 1.0 pm+/−either side of the reflected center wavelength 116E′.
The optical system 120 can include many sub-systems that use the narrowed light beam 124. Some of the subsystems can require both a selected maximum bandwidth and a selected minimum bandwidth. By way of example, the optical system 120 can include a scanner that requires several wavelengths centered on a selected wavelength and distributed across a bandwidth of sufficient breadth that can be used to generate a desired interference pattern.
In order to satisfy continuingly more stringent requirements to control bandwidth, particularly the width of the spectrum containing a selected percentage of the intensity, i.e., 95% (“E95%” or simply “E95”) or E95 separately from full width half maximum (“FWHM”) the need exists to distort the wavefront interaction surface of a center wavelength selection and bandwidth selection optical element (e.g., a dispersive grating having a plurality of dispersive optical features, e.g., grooves on one face thereof. These requirements can include a need for greater range of control as well as maintaining bandwidth within some small range and/or not to exceed some selected value. This distortion needs to be in two planes and needs to be independent in each of the two planes, with as little interference between the distorting mechanisms as possible and one distorting mechanism, such as the one distorting the separation of the groove forming features across the face of the dispersive optical element (as opposed to along the length of the dispersive optical element) has been found to need to be capable of exerting more distorting force. Applicants propose such modifications to existing laser system bandwidth control mechanisms.
In view of the foregoing, there is a need for a system, method and apparatus for bending an optical grating to select a bandwidth of wavelengths of light centered on a selected center wavelength and having a selected minimum bandwidth and a selected maximum bandwidth.